Be confident. While there is understandably a lot of stress involved in telling your mom that you are a lesbian, you do not want to tell her when you are in an certain state of mind. If you are confident when you tell her, she is more likely to be accepting.
Choose an appropriate time and place. Do not do it somewhere in public; you do not want your mother feeling embarrassed if she reacts emotionally. Make sure you will have plenty of time to talk with her after you tell her.
Try to be understanding. While you should be confident and happy with yourself, your mother may find it difficult to understand and be in shock or denial. Prepare yourself by expecting the worst, but do not too harshly judge your mom -- it may take her time to accept what you have told her.